1. Field of the Invention
The present invention generally relates to fluid pumps and more specifically relates to hydraulically driven diaphragm pumps.
2. Related Art
Hydraulically driven diaphragm pumps can be dived into at least two groups. The first group includes pumps that use a different stroke for the hydraulic piston or plunger than that of the diaphragm. These pumps can be referred to as asynchronous pumps. Asynchronous pumps are commonly used for metering in large diaphragm pumps where it is desirable to have a large diameter diaphragm that only defects a small amount (a “short stroke”). Short stroke diaphragms are typically driven by a much longer stroke hydraulic plunger or piston. The long stroke of the piston makes possible the use of a small diameter for the piston, which result in smaller loads on the crankshaft and crankcase that must move the piston through its stroke.
The second group includes pumps where the diaphragm center moves the same distance as the hydraulic piston. These pumps can be referred to as synchronous pumps. The diaphragm position in synchronous pumps is controlled by a valve in the piston that maintains a constant distance between the piston and diaphragm center.
An example valving system for diaphragm position control in synchronous pumps is disclosed in U.S. Pat. No. 3,884,598 (Wanner), which is incorporated herein by reference. Wanner discloses a system that senses the position of the diaphragm relative to the piston, and then functions to keep the position of the diaphragm constant. The Wanner system is useful for pumps that must operate at a high speed or that pump abrasive materials because the system permits the use of elastomeric diaphragms that do not need to come into contact with a stop surface at the end of stroke. However, if the piston travels more than the travel distance of the diaphragm, this system will not be able to properly maintain the amount of hydraulic fluid behind the diaphragm for the pump to function properly.
Some example asynchronous pumps are described in U.S. Pat. No. 5,246,351 (Horn), U.S. Pat. No. 5,667,368 (Augustyn), and U.S. Pat. No. 4,883,412 (Malizard). These example pumps all use a similar approach to diaphragm position control. Each of these pumps momentarily adjusts the amount of oil at the top or bottom of every stroke stroke. An overfill condition is detected when the diaphragm travels to far forward and reaches a limit of travel. This causes a higher than normal pressure of the hydraulic fluid, which causes a valve to momentarily open and release some of the excess fluid. This excess pressure is generated when the diaphragm reaches a stop, or simply the end point of deflection where higher pressure is required to move the diaphragm further. This pressure is not transmitted to the pumped fluid and therefore produces an unbalanced pressure drop across the diaphragm. This method of dealing with pressures created by overfill requires that the diaphragm includes materials and a configuration adequate to handle this unbalanced pressure without the diaphragm failing. This limitation on diaphragm materials and design results in the use of very large diameter, low deflection diaphragms that greatly increase the size and cost of the pump.
Known asynchronous hydraulically driven pumps do not allow for the use of highly flexible elastomeric diaphragms that are relatively small and capable of undergoing large deflections for at least those reasons discussed above. As a result, the use of these types of diaphragms is limited to synchronous pumps. The piston stroke in a synchronous pump must be relatively short since it is limited to the diaphragm stroke. This makes the crankshaft and crankcase bear the higher loads of a larger diameter piston, making the drive side of the pump more expensive.
Another example hydraulically driven pump is disclosed in U.S. Pat. No. 3,769,879 (Lofquist). Lofquist discloses a spool that moves with every stroke of the diaphragm to momentarily open ports between a fluid reservoir and the hydraulic chamber (e.g., transfer chamber) behind the diaphragm at the ends of the piston stoke. The ports and moving spool allow only a small pulse of fluid to pass with each stroke in order to correct an overfill or underfill condition.
Lofquist has some significant disadvantages under conditions of extreme underfill or overfill (e.g., conditions caused by very low or very high pump inlet pressure for the pumped fluid). Under extreme overfill conditions, the small pulse of fluid permitted with each stroke is insufficient to immediately correct the overfill, which results in stressing of the diaphragm until enough strokes occurred to correct the overfill condition. Another shortcoming of Lofquist relates to the direction in which the diaphragm is biased. Under extreme conditions (e.g., low inlet and outlet pressure for the pumped fluid caused by, for example, a blocked inlet to the pump), the Lofquist system tends to add oil to the transfer chamber without any bias applied to the diaphragm that would otherwise discharge the overfill of oil. As a result, the overfill cannot be solved and the diaphragm will fail.
There is a need therefore for a diaphragm position control that permits use of a highly flexible elastomeric diaphragms that are relatively small and capable of undergoing large deflections in both synchronous and asynchronous hydraulic pumps.